1. Field of the Invention
The present invention relates to a scattered ray removal grid used in a radiographic device and a method of producing the grid, particularly to a scattered ray removal grid formed to have spherical curvature.
2. Description of the Related Art
Scattered ray removal grids have been developed for use in X-ray imaging devices. Japanese Unexamined Patent Publication No. 2 (1990)-263279, for example, teaches the scattered ray removal grid 100 shown in FIG. 6. The scattered ray removal grid 100 is composed of radiation absorbers 102 and radiation non-absorbers 104 aligned alternately in a plate-like configuration. Of the rays, e.g., X-rays 108, emitted from a radiation source 113 and passing through a subject 106 (the transmitted rays) some are scattered in oblique directions by the subject 106 and become scattered rays 109 and others continue along substantially straight paths and become main transmitted X-rays 110. The scattered ray removal grid 100 absorbs and removes the scattered rays 109 and efficiently passes only the main transmitted X-rays 110. This reduces the amount of image noise caused by scattered rays. The radiation absorbers 102 of the scattered ray removal grid 100 are directed toward the radiation source (X-ray source) 444-113 and given tilt angles that increase with increasing proximity to the edges of the grid. This aligns them with advancing direction of the main transmitted X-rays 110 that pass through the subject 106 without scattering and therefore prevents decrease of the transmittance at the peripheral region.
The scattered ray removal grid 100 is, however, expensive to produce owing to the large number of fabrication steps made necessary by the need to vary the shape of the individual radiation non-absorbers 104 (formed of wood, aluminum or the like) in correspondence to the changing angle of the radiation absorbers 102 between which they fit.
The present invention was accomplished in light of the foregoing circumstances and has as an object to provide a scattered ray removal grid that is easy and inexpensive to produce. Another object of the present invention is to provide a method of producing the scattered ray removal grid.
The scattered ray removal grid according to the present invention is characterized in being formed in an overall shape of constant spherical curvature.
The method of producing a scattered ray removal grid according to the present invention comprises the steps of placing a plate-like grid having thermoplastic resin interposed between its grid elements between a set of dies having surfaces of complementary spherical curvature of prescribed radii, causing the set of dies to form the grid into a shape having spherical curvature of a prescribed radius by pressing and heating the grid to its softening temperature, and cooling the grid formed in a spherical shape.
The grid can be one whose radiation absorbers are arrayed in parallel or one whose radiation absorbers are arrayed in a lattice.
The method of producing a scattered ray removal grid according to the present invention heats a plate-like grid to a temperature and is composed of spaced grid elements made from a radiation-absorbing material and thermoplastic resin interposed between the grid elements, forms the grid into overall spherical shape, and then cools it. This method does not require the individual grid portions to be separately formed and can therefore produce the scattered ray removal grid with utmost ease and efficiency. The so-obtained scattered ray removal grid can therefore be produced with ease at low cost. It is therefore affordable enough to obtain in numbers for alignment in the advancing direction of the emitted X-rays at different distances from the X-ray source. Its low cost thus further expands its range of application. The X-ray sensor unit disposed behind the scattered ray removal grid is preferably given an approximately spherical curvature following that of the spherical scattered ray removal grid so as to prevent image distortion by ensuring that the transmitted rays enter the sensor substantially perpendicularly.